Receiving apparatus

ABSTRACT

A receiving apparatus includes: a local oscillator to output first- and second-local-oscillator signals whose phases are orthogonal to each other; a mixer to output first- and second-intermediate-frequency signals; a first filter to allow a component from a desired signal to pass therethrough, and eliminate a component from an image signal having a frequency symmetrical with that of the desired signal, in the first- and second-intermediate-frequency signals; a second filter to allow a component from the image signal to pass therethrough, and eliminate a component from the desired signal, in the first- and second-intermediate-frequency signals; a comparator to compare levels between output signals of the first and second filters; and a control unit to switch a frequency of the first- and second-local-oscillator signals to a difference frequency between a frequency of the desired signal and the intermediate frequency or a sum frequency thereof, according to a comparison result of the comparator.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority to Japanese PatentApplication No. 2009-286758, filed Dec. 17, 2009, of which full contentsare incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a receiving apparatus.

2. Description of the Related Art

In wireless communication, a received signal is generally demodulatedinto a baseband signal after being subjected to a filtering process, afrequency converting process, an amplifying process, etc. Particularly,a superheterodyne receiving apparatus is known in general that mixes areceived RF (Radio Frequency) signal with a local oscillator signal fromLO (Local Oscillator), to be converted into an IF (IntermediateFrequency) signal. The superheterodyne receiving apparatus does not needa circuit handling high frequency wave after the frequency convertingprocess from the RF signal to the IF signal.

The superheterodyne receiving apparatus receives not only a desiredsignal to be received but also an image signal having a frequency f2(=2fL−f1) symmetrical with a frequency f1 of the desired signal withrespect to a frequency fL of the local oscillator signal. Therefore, inorder to prevent interference, the image signal should be eliminatedwhen the received RF signals is converted into the IF signal.

For example, Japanese Laid-Open Patent Publication No. 2006-229619discloses a high-frequency circuit, in which a received RF signal ismixed with each of a pair of local oscillator signals having phasesorthogonal to each other and a pair of generated mixed signals arecombined after the phases thereof are shifted by ±45 degrees, so that animage signal is eliminated. The high-frequency circuit mixes the RFsignal and the local oscillator signals with a mixer using a Gilbertcell and the phases of the mixed signals are shifted with a polyphasefilter.

For example, Japanese Laid-Open Patent Publication No. 2008-167000discloses a receiving apparatus, in which a pair of mixed signalsgenerated in the same manner is input to a complex BPF (Band-PassFilter), so that an image signal is eliminated. The receiving apparatususes a complex BPF capable of fast response, so-called Gm-C filter,includes a transconductance amplifier and a capacitance element.

As such, in a superheterodyne receiving apparatus, image signals areeliminated so that the interference can be prevented, and thus only thedesired signals to be received can be received.

However, in the image signal eliminating method as described above, aphase shift and a gain deviation occurs in a pair of generated mixedsignals due to variability of parts used therein and change in ambienttemperature. The phase shift and the gain deviation causes the pair ofthe mixed signals not to have phases orthogonal to each other, or haveamplitudes different from each other, the image signal remains withoutbeing completely eliminated. Thus, the interference occurs between thedesired signal and the image signal, which deteriorates thecommunication quality of the receiving apparatus.

When a frequency of the IF signal is expressed by f0 (=|f1−fL|=|fL−f2|),a frequency difference Df between the desired signal and the imagesignal is as follows:

Df=|f1−f2|=2|fL−f2|=2f0.

Therefore, in the case of a low-IF receiving apparatus using thefrequency f0 of the IF signal on the order of several ten to severalhundred kHz, the frequency difference Df between the signals is small,which makes difficult to attenuate the image signals before thefrequency converting process. Therefore, the image signals remarkablyremain due to the phase shift and the gain deviation, particularly inthe low-IF receiving apparatus.

SUMMARY OF THE INVENTION

A receiving apparatus according to an aspect of the present invention,includes: a local oscillator configured to output first and second localoscillator signals whose phases are orthogonal to each other; a mixerconfigured to mix a received signal with the first local oscillatorsignal and output a first intermediate frequency signal, and to mix areceived signal with the second local oscillator signal and output asecond intermediate frequency signal, the first and second intermediatefrequency signals having a predetermined intermediate frequency; a firstfilter configured to allow a component from a desired signal to bereceived to pass therethrough, and eliminate a component from an imagesignal having a frequency symmetrical with a frequency of the desiredsignal with respect to a frequency of the first and second localoscillator signals, in the first and second intermediate frequencysignals; a second filter configured to allow a component from the imagesignal to pass therethrough, and eliminate a component from the desiredsignal, in the first and second intermediate frequency signals; acomparator configured to compare levels between output signals of thefirst and second filters; and a control unit configured to switch thefrequency of the first and second local oscillator signals to adifference frequency between the frequency of the desired signal and theintermediate frequency or to a sum frequency of the frequency of thedesired signal and the intermediate frequency, according to a comparisonresult of the comparator.

Other features of the present invention will become apparent fromdescriptions of this specification and of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For more thorough understanding of the present invention and advantagesthereof, the following description should be read in conjunction withthe accompanying drawings, in which:

FIG. 1 is a block diagram illustrating a configuration of a frequencyconverting unit according to a first embodiment of the presentinvention;

FIG. 2 is a block diagram illustrating an overall configuration of areceiving apparatus according to first and second embodiments of thepresent invention;

FIG. 3 is a circuit block diagram illustrating an example of a specificconfiguration of a phase inverter circuit 41;

FIG. 4 is a diagram for explaining operations of filters 51 and 52;

FIG. 5 is a schematic diagram illustrating an example of a relationshipbetween a desired signal and an image signal in low side injection wherea frequency fL of local oscillator signals is lower than a frequency f1of a desired signal;

FIG. 6 is a schematic diagram illustrating an example of a relationshipbetween a desired signal and an image signal in high side injectionwhere a frequency fL of local oscillator signals is higher than afrequency f1 of a desired signal; and

FIG. 7 is a block diagram illustrating a configuration of a frequencyconverting unit according to a second embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

At least the following details will become apparent from descriptions ofthis specification and of the accompanying drawings.

First Embodiment Overall Configuration of Receiving Apparatus

An overall configuration of a receiving apparatus according to a firstembodiment of the present invention will hereinafter be described withreference to FIG. 2.

The receiving apparatus depicted in FIG. 2 includes an antenna 1, an RFamplifying unit 2, a frequency converting unit 3, an IF amplifying unit6, a demodulating unit 7, an audio processing unit 8, and a speaker 9.The receiving apparatus is used for receiving an FM radio broadcast oran AM radio broadcast, for example.

An RF signal output from the antenna 1 is input to the RF amplifyingunit 2, and an RFa signal output from the RF amplifying unit 2 is inputto the frequency converting unit 3. An IF signal output from thefrequency converting unit 3 is input to the IF amplifying unit 6, and anIFa signal output from the IF amplifying unit 6 is input to thedemodulating unit 7. Further, an AF (audio frequency) signal output fromthe demodulating unit 7 is input to the audio processing unit 8, and anAFa signal output from the audio processing unit 8 is input to thespeaker 9. On the other hand, a tuning signal TN is also input to thefrequency converting unit 3, and a frequency switching signal FS outputfrom the frequency converting unit 3 is input to the audio processingunit 8.

==Overall Operation of Receiving Apparatus==

An overall operation of the receiving apparatus according to anembodiment of the present invention will then be described.

The antenna 1 receives a broadcast wave of the FM radio broadcast or theAM radio broadcast, for example, and outputs the RF signal. The RFamplifying unit 2 selectively amplifies a frequency band of a desiredsignal to be received which is included in the RF signal and outputs theRFa signal. The frequency converting unit 3 converts a frequency of theRFa signal and outputs the IF signal after eliminating an image signal,etc., as needed using a complex BPF, etc. An operation of the frequencyconverting unit 3 will be described later in detail.

Although, for example, a typical FM radio receiving apparatus uses afrequency of 10.7 MHz, etc., as a frequency f0 of the IF signal, thereceiving apparatus according to an embodiment of the present inventionhas a configuration which is preferable especially for a low-IF modeusing the frequency f0 of the IF signal of the order of several ten toseveral hundred kHz. The IF signal is output as an analogue signal or adigital signal depending on a configuration subsequent to the IFamplifying unit 6.

The IF amplifying unit 6 amplifies the IF signal as needed according toa reception condition, and outputs the IFa signal. For example, the IFamplifying unit 6 includes an AGC (Automatic Gain Control) circuit thatperforms amplification with a gain corresponding to signal intensity ofthe IF signal and an IF filter that is a BPF whose passband widthvarying depending on the presence or absence of a disturbing signal suchas adjacent disturbance and multipath disturbance. The demodulating unit7 demodulates the IFa signal and outputs an AF signal.

The audio processing unit 8 controls sound volume and sound quality ofthe AF signal according to a reception condition and output the AFasignal. For example, the audio processing unit 8 includes a stereodemodulating unit that demodulates the AF signal into a stereo signalwith stereo separation (degree of separation) corresponding to areception condition, and a LPF (low-pass filter) that eliminates fromthe AF signal a component greater than or equal to a cutoff frequencycorresponding to a reception condition. The audio processing unit 8mutes the AFa signal when a frequency fL of a local oscillator signal isswitched in the frequency converting unit 3, based on the frequencyswitching signal FS. The speaker 9 converts the AFa signal into sound,to be output.

==Configuration of Frequency Converting Unit==

A configuration of the frequency converting unit according to anembodiment of the present invention will hereinafter be described withreference to FIG. 1. In an embodiment of the present invention, in-phasesignals I1 to I4 correspond to first intermediate frequency signals andquadrature signals Q1 to Q4 correspond to second intermediate frequencysignals.

A frequency converting unit 3 a depicted in FIG. 1 includes an LOcontrol unit 31 a, an LO 32, mixers 33 and 34, a phase inverter circuit41, filters 51 and 52, adders 53 and 54, and a comparator 55.

The tuning signal TN is input to the LO control unit 31 a, and an LOcontrol signal LC output from the LO control unit 31 a is input to theLO 32. The LO 32 outputs a (first) local oscillator signal L1 and a(second) local oscillator signal L2. On the other hand, the LO controlunit 31 a also outputs the frequency switching signal FS.

The mixers 33 and 34 are made up using a Gilbert cell as depicted inFIG. 6 of Japanese Laid-Open Patent Publication No. 2006-229619, forexample. The RFa signal and the local oscillator signal L1 are input tothe mixer 33, and the in-phase signal I1 is output from the mixer 33. Onthe other hand, the RFa signal and the local oscillator signal L2 areinput to the mixer 34, and the quadrature signal Q1 is output from themixer 34. The in-phase signal I1 and the quadrature signal Q1 are inputto the phase inverter circuit 41, and the in-phase signal I2 and thequadrature signal Q2 are output from the phase inverter circuit 41. Aconfiguration of the phase inverter circuit 41 will be described laterin detail.

The filters 51 and 52 are complex BPFs (Gm-C filters) including atransconductance amplifier and a capacitance element, as depicted inFIG. 4 of Japanese Laid-Open Patent Publication No. 2008-167000, forexample. To the filter 51, the in-phase signal I2 is input as a realpart of a complex signal and the quadrature signal Q2 is input as animaginary part of the complex signal, and the filter 51 outputs thein-phase signal I3 and the quadrature signal Q3. On the other hand, tothe filter 52, the quadrature signal Q2 is input as a real part of acomplex signal and the in-phase signal I2 is input as an imaginary partof the complex signal, and the filter 52 outputs the in-phase signal I4and the quadrature signal Q4.

In an embodiment of the present invention, the filters 51 and 52 havethe same characteristics. As will be described later, in an embodimentof the present invention, the filter 51 allows a desired signalcomponent to pass therethrough and eliminates an image signal component,and always corresponds to a first filter. On the other hand the filter52 allows the image signal component to pass therethrough and eliminatesthe desired signal component, and always corresponds to a second filter.

The in-phase signal I3 and the quadrature signal Q3 are input to theadder 53, and an IF1 signal is output from the adder 53. The frequencyconverting unit 3 a always outputs the IF1 signal as the IF signal. Onthe other hand, the in-phase signal 14 and the quadrature signal Q4 areinput to the adder 54, and an IF2 signal is output from the adder 54.Further, the IF1 signal is input to a non-inverting input of thecomparator 55, the IF2 signal is input to an inverting input thereof,and a comparison result signal CP output from the comparator 55 isreturned to the LO control unit 31 a and the phase inverter circuit 41.

==Example of Configuration of Phase Inverter Circuit==

A configuration of the phase inverter circuit 41 will hereinafter bedescribed with reference to FIG. 3.

The phase inverter circuit 41 includes resistors R1 to R10, (NPN)transistors T1 to T6, current sources S1, S2, and a switch circuit SW,for example.

The positive side (I1 p) and the negative side (I1 n) of the in-phasesignal I1 are input to the bases of the transistors T1 and T2,respectively. One ends of the resistors R2 and R4 are connected to theemitters of the transistors T1 and T2, respectively, and the other endsthereof are both supplied with sink currents from the current source S1which is connected to a ground potential. One ends of the resistors R1and R3 are connected to the collectors of the transistors T1 and T2,respectively, and the other ends thereof are both connected to a powersupply potential VCC. A connection point between the transistor T1 andthe resistor R1 and a connection point between the transistor T2 and theresistor R3 are output nodes on the positive side (I2 p) and thenegative side (I2 n) of the in-phase signal I2, respectively.

The positive side (Q1 p) and the negative side (Q1 n) of the quadraturesignal Q1 are input to the bases of the transistors T3 and T4,respectively. One ends of the resistors R6 and R8 are connected to theemitters of the transistors T3 and T4, respectively, and the other endsthereof are both supplied with sink currents via the switch SW from thecurrent source S2 which is connected to the ground potential. One endsof the resistors R5 and R7 are connected to the collectors of thetransistors T3 and T4, respectively, and the other ends thereof are bothconnected to the power supply potential VCC. A connection point betweenthe transistor T3 and the resistor R5 and a connection point between thetransistor T4 and the resistor R7 are output nodes on the positive side(Q2 p) and the negative side (Q2 n) of the quadrature signal Q2,respectively.

The positive side and the negative side of the quadrature signal Q1 areinput to the bases of the transistors T5 and T6, respectively. One endsof the resistors R9 and R10 are connected to the emitters of thetransistors T5 and T6, respectively, and the other ends thereof are bothsupplied with a sink current via the switch SW from the current sourceS2. The collector of the transistor T5 is connected to the output nodeon the negative side of the quadrature signal Q2 and the collector ofthe transistor T6 is connected to the output node on the positive sideof the quadrature signal Q2.

As described above, depending on the state of the switch SW, the sinkcurrent supplied from the current source S2 is supplied to thetransistors T3 and T4 via the resistors R6 and R8, or is supplied to thetransistors T5 and T6 via the resistors R9 and R10. Therefore, comparinga connection state of the switch SW indicated by a solid line and aconnection state of a short dashed line of FIG. 3, the phase of thequadrature signal Q2 to be output is inverted. Therefore, an inversionoccurs according to the state of the switch SW, in the phaserelationship between the in-phase signal I2 and the quadrature signalQ2, i.e., whether the phase of the quadrature signal Q2 is advanced ordelayed by 90 degrees relative to the in-phase signal I2. The switch SWis controlled by the comparison result signal CP as will be describedlater.

==Operation of Frequency Converting Unit==

An operation of the frequency converting unit 3 a according to anembodiment of the present invention will then be described. In thefollowing description, a desired signal and an image signal included inthe RFa signal are represented by H1 and H2, respectively.

The operation will first be described for the case of low side injectionwhere a frequency fL of the local oscillator signals L1 and L2 is lowerthan the frequency f1 of the desired signal H1. By way of example,assuming that the frequency f1 is 100 MHz and the frequency f0 of the IFsignal is 200 kHz, the frequency fL is 99.8 MHz and the frequency f2 ofthe image signal H2 is 99.6 MHz, as depicted in FIG. 5. Therefore, afrequency difference Df between the desired signal H1 and the imagesignal H2 is only 400 kHz, and it is difficult to sufficiently attenuatethe image signal H2 in the antenna 1 and the RF amplifying unit 2.

The desired signal H1 and the image signal H2 can respectively beexpressed as follows:

H1=A·sin(ω1·t); and

H2=B·sin(ω2·t),

where an amplitude and an angular frequency of the desired signal H1 areA and ω1 (=2π×f1), respectively, and an amplitude and an angularfrequency of the image signal H2 are B and ω2 (=2π×f2), respectively.Since the IF amplifying unit 6 and the demodulating unit 7 can eliminatecomponents other than the desired signal H1 and the image signal H2included in the RFa signal, it is assumed that RFa=H1+H2 is satisfied inthe following description.

The LO control unit 31 a calculates the frequency fL in accordance withthe frequency f1 corresponding to the tuning signal TN, and outputs theLO control signal LC for controlling the LO 32 such that the frequencyof the local oscillator signals L1 and L2 is set at the calculatedfrequency fL. The frequency fL in the low side injection satisfiesfL=f1−f0.

The LO 32 outputs the local oscillator signals L1 and L2 whose phasesorthogonal to each other. Assuming that the local oscillator signal L2is advanced 90 degrees in phase as compared to the signal L1 and thatthe angle frequency of the local oscillator signals L1 and L2 is ωL(=2π×fL), the local oscillator signals L1 and L2 can be respectivelyexpressed as follows:

L1=sin(ωL·t); and

L2=cos(ωL·t).

The mixer 33 mixes the RFa signal with the local oscillator signal L1,and outputs the in-phase signal I1. Although the output signal of themixer 33 contains frequency components of sums of the frequencies f1, f2and the frequency fL (f1+fL, f2+fL), the frequency components of thesums can be eliminated by the filters 51 and 52 and the demodulatingunit 7. Therefore, only a frequency component of a difference betweenthe frequencies f1, f2 and the frequency fL (f1−fL=fL−f2=f0) isconsidered in the in-phase signal I1, and thus the in-phase signal I1can be expressed as follows:

$\begin{matrix}{{I\; 1} = {{Rfa} \times L\; 1}} \\{= {{\left( {A/2} \right) \cdot {\cos \left( {\omega \; {0 \cdot t}} \right)}} + {\left( {B/2} \right) \cdot {\cos \left( {\omega \; {0 \cdot t}} \right)}}}}\end{matrix}$

where the angle frequency of the IF signal is ω0 (=2π×f0).

The mixer 34 mixes the RFa signal with the local oscillator signal L2and outputs the quadrature signal Q1. Therefore, when calculation ismade in the same manner as in the case of the in-phase signal I1, thequadrature signal Q1 can be expressed as follows:

$\begin{matrix}{{Q\; 1} = {{Rfa} \times L\; 2}} \\{= {{\left( {A/2} \right) \cdot {\cos \left( {\omega \; {0 \cdot t}} \right)}} + {\left( {B/2} \right) \cdot {{\cos \left( {\omega \; {0 \cdot t}} \right)}.}}}}\end{matrix}$

As described above, the phase inverter circuit 41 inverts the phase ofthe quadrature signal Q1 according to the comparison result signal CP.In the case of the low side injection, assuming that the connectionstate of the switch SW of FIG. 3 is as indicated by the solid line, thephase relationship between the in-phase signal I1 and the quadraturesignal Q1 is not changed by the phase inverter circuit 41. Therefore, itis considered that I2=I1 and Q2=Q1 are satisfied.

To the filter 51 which is a complex BPF, the in-phase signal I2 is inputas a real part of a complex signal and the quadrature signal Q2 is inputas an imaginary part of the complex signal, as described above.Therefore, when an imaginary unit is represented by j, the complexsignal input to the filter 51 can be expressed as follows:

$\begin{matrix}{{{I\; 2} + {j\; Q\; 2}} = {{I\; 1} + {j\; Q\; 1}}} \\{= {{\left( {A/2} \right) \cdot {\exp \left( {{j \cdot \omega}\; {0 \cdot t}} \right)}} +}} \\{{\left( {B/2} \right) \cdot {{\exp \left( {{{- j} \cdot \omega}\; {0 \cdot t}} \right)}.}}}\end{matrix}$

For example, as depicted in FIG. 4, the filter 51 allows only a positivefrequency component with the frequency f0 centered to pass therethroughand cuts off a negative frequency component. In FIG. 4, by way ofexample, a bandwidth is set at 180 kHz. Therefore, the complex signaloutput from the filter 51 is expressed as

I3+jQ3=(A/2)·exp(j·ω0·t),

and the image signal component is eliminated. The adder 53 adds thein-phase signal I3 and the quadrature signal Q3, and outputs the IF1signal containing the desired signal component.

On the other hand, to the filter 52 which is a complex BPF, thequadrature signal Q2 is input as a real part of a complex signal and thein-phase signal I2 is input as an imaginary part of the complex signal,as described above. Therefore, the complex signal input to the filter 52can be expressed as follows:

$\begin{matrix}{{{Q\; 2} + {j\; I\; 2}} = {{Q\; 1} + {j\; I\; 1}}} \\{= {j \cdot \left( {{I\; 1} - {j\; Q\; 1}} \right)}} \\{= {{\left( {A/2} \right) \cdot j \cdot {\exp \left( {{{- j} \cdot \omega}\; {0 \cdot t}} \right)}} +}} \\{{\left( {B/2} \right) \cdot j \cdot {{\exp \left( {{j \cdot \omega}\; {0 \cdot t}} \right)}.}}}\end{matrix}$

Since the filter 52 has the same characteristics as the filter 51 asdescribed above, the complex signal output from the filter 52 isexpressed as

I4+jQ4=(B/2)·j·exp(j·ω0·t),

and the desired signal component is eliminated. The adder 54 adds thein-phase signal I4 and the quadrature signal Q4, and outputs the IF2signal containing the image signal component.

The comparator 55 compares the levels between the IF1 signal and the IF2signal, and outputs the comparison result signal CP. The comparisonresult signal CP goes low when the IF2 signal is higher in level thanthe IF1 signal.

As described above, the frequency converting unit 3 a can eliminate theimage signal component with the filter 51, and output the IF1 signal asthe IF signal. However, if a phase shift or a gain deviation occurs inthe in-phase signal I2 and the quadrature signal Q2, the image signalcomponent is not completely eliminated, and remains in the IF1 signal.As an amplitude B of the image signal H2 is increased relative toamplitude A of the desired signal H1, the image signal component remainsin the IF1 signal at a higher rate, which may generate interference.

In an embodiment of the present invention, for example, if thecomparison result signal CP goes low in the case of the low sideinjection, switching to high side injection is performed where thefrequency fL of the local oscillator signals L1 and L2 is higher thanthe frequency f1 of the desired signal H1, so that the interference isprevented. In such case, as depicted in FIG. 6, the frequency fL is setat 100.2 MHz and the frequency f2 of the image signal H2 is set at 100.4MHz. Therefore, the image signal in the case of the low side injection,i.e., the signal having a frequency of 99.6 MHz, does not interfere withthe desired signal H1. A description will hereinafter be given of aspecific operation in the case of switching from the low side injectionto the high side injection.

As described above, in the low side injection, the LO control unit 31 aoutputs such LO control signal LC as to satisfy fL=f1−f0. When thecomparison result signal CP goes low, the LO control unit 31 a outputssuch LO control signal LC as to satisfy fL=f1+f0, and switching to thehigh side injection is performed.

As is the case with the low side injection, the mixers 33 and 34 mix theRfa signals with the local oscillator signals L1 and L2, respectively,and output the in-phase signal I1 and the quadrature signal Q1,respectively. On the other hand, in the high side injection, since afrequency difference between the frequencies f1 and f2 and the frequencyfL is fL−f1=f2−fL=f0, the in-phase signal I1 and the quadrature signalQ1 can be respectively expressed as follows:

I1=(A/2)·cos(ω0·t)+(B/2)·cos(ω0·t); and

Q1=−(A/2)·cos(ω0·t)+(B/2)·cos(ω0·t).

As described above, in the case of the low side injection, the phaseinverter circuit 41 has the connection state of the switch SW of FIG. 3as indicated by the solid line, and the phase relationship between thein-phase signal I1 and the quadrature signal Q1 is not changed. When thecomparison result signal CP goes low, the phase inverter circuit 41switches the connection state of the switch SW of FIG. 3 to such a stateas indicated by the short dashed line, and inverts the phase of thequadrature signal Q1. Therefore, in the high side injection, it isconsidered that I2=I1 and Q2=−Q1 are satisfied.

As is the case with the low side injection, the in-phase signal I2 andthe quadrature signal Q2 are input as complex signals to the filters 51and 52 that are complex BPFs, and the complex signals input to thefilters 51 and 52 can respectively be expressed as follows:

$\begin{matrix}{{{I\; 2} + {j\; Q\; 2}} = {{I\; 1} - {j\; Q\; 1}}} \\{= {{\left( {A/2} \right) \cdot {\exp \left( {{j \cdot \omega}\; {0 \cdot t}} \right)}} +}} \\{{{\left( {B/2} \right) \cdot {\exp \left( {{{- j} \cdot \omega}\; {0 \cdot t}} \right)}};}}\end{matrix}$ and $\begin{matrix}{{{Q\; 2} + {j\; I\; 2}} = {{{- Q}\; 1} + {j\; I\; 1}}} \\{= {j \cdot \left( {{I\; 1} + {j\; Q\; 1}} \right)}} \\{= {{\left( {A/2} \right) \cdot j \cdot {\exp \left( {{{- j} \cdot \omega}\; {0 \cdot t}} \right)}} +}} \\{{\left( {B/2} \right) \cdot j \cdot {{\exp \left( {{j \cdot \omega}\; {0 \cdot t}} \right)}.}}}\end{matrix}$

The complex signals output from the filters 51 and 52 are respectivelyexpressed as follows:

I3+jQ3=(A/2)·exp(j·ω0·t); and

I4+jQ4=(B/2)·j·exp(j·ω0·t).

Therefore, as is the case with the low side injection, the filters 51and 52 eliminate the image signal component and the desired signalcomponent, respectively, and the adders 53 and 54 output the IF1 signalcontaining the desired signal component and the IF2 signal containingthe image signal component, respectively.

As described above, the comparator 55 compares the levels between theIF1 signal and the IF2 signal, and outputs the comparison result signalCP which goes low when the IF2 signal is higher in level than the IF1signal.

On the other hand, if the comparison result signal CP goes low in thehigh side injection, the LO control unit 31 a outputs such LO controlsignal LC as to satisfy fL=f1−f0, and switching to the low sideinjection is performed. The phase inverter circuit 41 switches theconnection state of the switch SW of FIG. 3 to such a state as indicatedby the solid line, and does not invert the phase of the quadraturesignal Q1.

As such, in the frequency converting unit 3 a according to an embodimentof the present invention, if the IF2 signal is higher in level than theIF1 signal, switching is performed between the low side injection(fL=f1−f0) and the high side injection (fL=f1+f0), so that theinterference can be prevented.

The LO control unit 31 a outputs the frequency switching signal FS thatgoes high when switching is performed between the low side injection andthe high side injection, i.e., when the frequency fL is switched, forexample. The audio processing unit 8 can prevent noise from being outputfrom the speaker 9 at the time of switching of the frequency fL, bymuting the AFa signal while the frequency switching signal FS is high.

Second Embodiment Configuration of Frequency Converting Unit

Overall configuration and an operation of the receiving apparatusaccording to a second embodiment of the present invention are the sameas those of the receiving apparatus according to a first embodiment ofthe present invention.

A configuration of the frequency converting unit according to anembodiment of the present invention will hereinafter be described withreference to FIG. 7. In this embodiment of the present invention, thein-phase signals I1, I3, and I4 correspond to the first intermediatefrequency signals and the quadrature signals Q1, Q3, and Q4 correspondto the second intermediate frequency signals.

A frequency converting unit 3 b depicted in FIG. 7 includes an LOcontrol unit 31 b in place of the LO control unit 31 a, and amultiplexer (selection circuit) 42 in place of the phase invertercircuit 41, as compared to the frequency converting unit 3 a accordingto a first embodiment of the present invention.

The tuning signal TN is input to the LO control unit 31 b, and the LOcontrol signal LC output from the LO control unit 31 b is input to theLO 32. The LO 32 outputs the local oscillator signals L1 and L2. On theother hand, the LO control unit 31 b also outputs the frequencyswitching signal FS and an up/down selecting signal UL.

As is the case with a first embodiment of the present invention, the RFasignal and the local oscillator signal L1 are input to the mixer 33, theRFa signal and the local oscillator signal L2 are input to the mixer 34,and the mixers 33 and 34 output the in-phase signal I1 and thequadrature signal Q1, respectively. In an embodiment of the presentinvention, the in-phase signal I1 and the quadrature signal Q1 aredirectly input to the filters 51 and 52.

To the filter 51, the in-phase signal I1 is input as a real part of acomplex signal and the quadrature signal Q1 is input as an imaginarypart of the complex signal, and the filter 51 outputs the in-phasesignal I3 and the quadrature signal Q3. On the other hand, to the filter52, the quadrature signal Q1 is input as a real part of a complexsignal, and the in-phase signal I1 is input as an imaginary part of thecomplex signal, and the filter 52 outputs the in-phase signal I4 and thequadrature signal Q4.

As is the case with a first embodiment of the present invention, in thisembodiment of the present invention as well, the filters 51 and 52 havethe same characteristics. As will be described later, in this embodimentof the present invention, filters corresponding to the first and secondfilters are interchanged depending on the cases of the low sideinjection and the high side injection.

The in-phase signal I3 and the quadrature signal Q3 are input to theadder 53, and an IF3 signal is output from the adder 53. On the otherhand, the in-phase signal I4 and the quadrature signal Q4 are input tothe adder 54, and an IF4 signal is output from the adder 54. The IF3signal is input to the non-inverting input of the comparator 55, the IF4signal is input to the inverting input thereof, and the comparisonresult signal CP output from the comparator 55 is returned to the LOcontrol unit 31 b. The IF3 signal and the IF4 signal are input to datainputs of the multiplexer 42, and the up/down selecting signal UL isinput to a selection control input. The IF signal output from themultiplexer 42 is output from the frequency converting unit 3 b.

==Operation of Frequency Converting Unit==

An operation of the frequency converting unit 3 b according to anembodiment of the present invention will then be described.

The operation will first be described for the case of the low sideinjection.

In a first embodiment of the present invention, in the case of the lowside injection, the phase relationship between the in-phase signal I1and the quadrant signal Q1 is not changed by the phase inverter circuit41. Therefore, in this embodiment of the present invention, as is thecase with a first embodiment of the present invention, the image signalcomponent is eliminated by the filter 51 corresponding to the firstfilter, and the desired signal component is eliminated by the filter 52corresponding to the second filter. Therefore, the IF3 signal and theIF4 signal in this embodiment of the present invention are equivalent tothe IF1 signal and the IF2 signal in a first embodiment of the presentinvention. The comparison result signal CP goes low when the IF4 signalis higher in level than the IF3 signal.

The multiplexer 42 selects and outputs either one of the IF3 signal andthe IF4 signal as the IF signal according to the up/down selectingsignal UL which is indicative of the current frequency fL. Morespecifically, the up/down selecting signal UL indicates the high sideinjection or the low side injection, and goes high in the case of thehigh side injection and goes low in the case of the low side injection,for example. In the case of the low side injection, the multiplexer 42outputs the IF3 signal as the IF signal.

A description will then be given of the operation in the case ofswitching from the low side injection to the high side injection.

In the low side injection, the LO control unit 31 b outputs such LOcontrol signal LC as to satisfy fL=f1−f0. When the comparison resultsignal CP goes low, the LO control unit 31 b outputs such LO controlsignal LC as to satisfy fL=f1+f0, and switching to the high sideinjection is performed.

As described above, in this embodiment of the present invention, sincethe in-phase signal I1 and the quadrant signal Q1 are directly input tothe filters 51 and 52, the complex signals input to the filters 51 and52 can respectively be expressed as follows:

I 1 + j Q 1 = (A/2) ⋅ exp (−j ⋅ ω 0 ⋅ t) + (B/2) ⋅ exp (j ⋅ ω 0 ⋅ t);and $\begin{matrix}{{{Q\; 1} + {j\; I\; 1}} = {j \cdot \left( {{I\; 1} - {j\; Q\; 1}} \right)}} \\{= {{\left( {A/2} \right) \cdot j \cdot {\exp \left( {{j \cdot \omega}\; {0 \cdot t}} \right)}} +}} \\{{\left( {B/2} \right) \cdot j \cdot {{\exp \left( {{{- j} \cdot \omega}\; {0 \cdot t}} \right)}.}}}\end{matrix}$

The complex signals output from the filters 51 and 52 are respectivelyexpressed as follows:

I3+jQ3=(B/2)·exp(j·ω0·t); and

I4+jQ4=(A/2)·j·exp(j·ω0·t).

Therefore, the image signal component is eliminated by the filter 52,and the desired signal component is eliminated by the filter 51. In thecase of the high side injection, the filter 52 corresponds to the firstfilter, and the filter 51 corresponds to the second filter. The adder 53outputs the IF3 signal containing the image signal component, and theadder 54 outputs the IF4 signal containing the desired signal component.In the case of the high side injection, the multiplexer 42 outputs theIF4 signal as the IF signal.

On the other hand, in the case of the high side injection, when thecomparison result signal CP goes high, i.e., when the IF3 signal becomeshigher in level than the IF4 signal, the LO control unit 31 b performsswitching to the low side injection.

As such, in the frequency converting unit 3 b according to thisembodiment of the present invention, switching is performed between thelow side injection and the high side injection according to thecomparison result signal CP and according to whether the presentheterodyne is the high side injection or the low side injection, so thatthe interference can be prevented.

As described above, in the frequency converting unit 3, switching isperformed between the low side injection and the high side injectionaccording to a comparison result between the output signal level of thefirst filter which eliminates the image signal component from thein-phase signal and the quadrant signal and the output signal level ofthe second filter which eliminates the desired signal componenttherefrom, so that the interference between the desired signal H1 andthe image signal H2 can be prevented even if a phase shift or a gaindeviation occurs.

The first and second filters are configured as complex BPFs to which thein-phase signals and the quadrant signals are input as complex signals,and only either one of the positive and negative frequency components ofthe input signal is allow to pass therethrough so that the image signalcomponent or the desired signal component can be removed.

In the frequency converting unit 3 a, the quadrature signal Q1 isinverted in phase according to the comparison result signal CP, and thein-phase signal I2 and the quadrature signal Q2 are interchanged to beinput to the filter 52, so that the image signal component and thedesired signal component can be respectively eliminated using thefilters 51 and 52 with the same characteristics.

In the frequency converting unit 3 b, switching is performed between thelow side injection and the high side injection according to thecomparison result signal CP and according to whether the currentheterodyne is the high side injection or the low side injection, so thatthe image signal component and the desired signal component can beeliminated without using a phase inverter circuit using the filters 51and 52 having the same characteristics.

the frequency switching signal FS is output which goes high whenswitching is performed between the low side injection and the high sideinjection, and the AFa signal is muted while the frequency switchingsignal FS is high, so that the noise can be prevented from being outputfrom the speaker 9 at the time of switching of the frequency fL.

In the above described embodiments of the present invention, the filters51 and 52 have the same characteristics and the in-phase signal and thequadrature signal are interchanged and input to the filter 52, but thisis not a limitative. For example, a configuration may be such that thein-phase signal and the quadrature signal is input to both of thefilters in the same manner, one filter allows only a positive frequencycomponent (portion of a solid line of FIG. 4) to pass therethrough, andthe other filter allows only a negative frequency component (portion ofa short dashed line of FIG. 4) to pass therethrough. In such case,either one of the filters can eliminate the image signal component, andthe other one of the filters can eliminate the desired signal component.

In the above described embodiments, the frequency switching signal FSgoes high when switching is performed between the low side injection andthe high side injection, but this is not a limitative. Further, thefrequency switching signal FS may also be set to high, when thefrequency fL fluctuates in association with change in the frequency f1corresponding to the tuning signal TN.

The above embodiments of the present invention are simply forfacilitating the understanding of the present invention and are not inany way to be construed as limiting the present invention. The presentinvention may variously be changed or altered without departing from itsspirit and encompass equivalents thereof.

1. A receiving apparatus comprising: a local oscillator configured tooutput first and second local oscillator signals whose phases areorthogonal to each other; a mixer configured to mix a received signalwith the first local oscillator signal and output a first intermediatefrequency signal, and to mix a received signal with the second localoscillator signal and output a second intermediate frequency signal, thefirst and second intermediate frequency signals having a predeterminedintermediate frequency; a first filter configured to allow a componentfrom a desired signal to be received to pass therethrough, and eliminatea component from an image signal having a frequency symmetrical with afrequency of the desired signal with respect to a frequency of the firstand second local oscillator signals, in the first and secondintermediate frequency signals; a second filter configured to allow acomponent from the image signal to pass therethrough, and eliminate acomponent from the desired signal, in the first and second intermediatefrequency signals; a comparator configured to compare levels betweenoutput signals of the first and second filters; and a control unitconfigured to switch the frequency of the first and second localoscillator signals to a difference frequency between the frequency ofthe desired signal and the intermediate frequency or to a sum frequencyof the frequency of the desired signal and the intermediate frequency,according to a comparison result of the comparator.
 2. The receivingapparatus of claim 1, wherein the first and second filters are a pair ofcomplex filters configured to be input with the first and secondintermediate frequency signals as a complex signal, and allow either oneof positive and negative frequency components of an input signal to passtherethrough.
 3. The receiving apparatus of claim 2, further comprisinga phase inverter circuit configured to invert a phase of either one ofthe first intermediate frequency signal and the second intermediatefrequency signal according to a comparison result of the comparator,wherein the first intermediate frequency signal and the secondintermediate frequency signal are input to the first filter as a realpart of the complex signal and an imaginary part of the complex signal,respectively, and either one of the positive and negative frequencycomponents of the input signal is allowed by the first filter to passtherethrough, and wherein the first intermediate frequency signal andthe second intermediate frequency signal are input to the second filteras an imaginary part of the complex signal and a real part of thecomplex signal, respectively, and the same frequency component of theinput signal as the frequency component in the first filter is allowedby the second filter to pass therethrough.
 4. The receiving apparatus ofclaim 2, further comprising a selection circuit configured to select andoutput either one of output signals of the pair of the complex filtersaccording to a current frequency of the first and second localoscillator signals, wherein the control unit switches the frequency ofthe first and second local oscillator signals according to a comparisonresult of the comparator and the current frequency of the first andsecond local oscillator signals.
 5. The receiving apparatus of claim 1,further comprising a demodulating unit configured to demodulate anoutput signal of the first filter into an audio signal, and an audioprocessing unit configured to mute the audio signal when the frequencyof the first and second local oscillator signals is switched.
 6. Thereceiving apparatus of claim 2, further comprising a demodulating unitconfigured to demodulate an output signal of the first filter into anaudio signal, and an audio processing unit configured to mute the audiosignal when the frequency of the first and second local oscillatorsignals is switched.
 7. The receiving apparatus of claim 3, furthercomprising a demodulating unit configured to demodulate an output signalof the first filter into an audio signal, and an audio processing unitconfigured to mute the audio signal when the frequency of the first andsecond local oscillator signals is switched.
 8. The receiving apparatusof claim 4, further comprising a demodulating unit configured todemodulate an output signal of the first filter into an audio signal,and an audio processing unit configured to mute the audio signal whenthe frequency of the first and second local oscillator signals isswitched.